I've got to do a bit of patching on a wall where I removed a cupboard. Previously I've used plaster undercoat but having had some plastering done a few months ago I notice they used a sand and cement mix rather than the browning stuff as it is less affected by damp.
What is the correct mix of sand and cement for undercoat when plastering?
If I mix it myself, I do 1:1:6 cement:lime:sand. I did a load over Easter for a friend, and I couldn't be bothered to mix it myself, so I just bought 4 bags of ready-mixed mortar. I would guess this is 1:4 or 1:5 cement:sand, but it didn't say. Worked fine.
If you want it waterproof, buy a mortar waterproofer to add to the mix (or better still, a combined waterproofer/plasticiser). This works very well -- I've rendered damp walls with it and you can use standard gypsom finish coat plaster on top without it getting damp at all. If you are doing multiple coats, only put the waterproofer in one of the coats or it may have problems drying out. Wait 24-48 hours between coats and before applying finish coat plaster (mortar wants to be initial set but still damp). Note that mortar takes weeks to reach final set, and the surface (if you are leaving the mortar exposed) is easily marked during this period.
and if you are not leaving it exposed, then scratching it up a bit before it sets can help provide a key for the skim coat...
Yes. Do this for all coats except the final one. Do it before it reaches initial set.
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